As is known, the petroleum industry routinely uses cracking processes in which hydrocarbon molecules of high molecular weight and a high boiling point are broken down into smaller molecules boiling in lower temperature ranges suitable for the desired use.
The process which today is used most widely for this purpose is the so-called fluid catalytic cracking (FCC) process. In this type of process, the hydrocarbon feedstock is vaporized by being contacted at high temperature with a cracking catalyst that is kept in suspension in the feedstock vapors. After the desired molecular-weight range reduction has been attained by cracking, with a corresponding lowering of the boiling points, the catalyst is separated from the products obtained, stripped, regenerated by combustion of the coke formed, and again contacted with the feedstock to be cracked.
In processes of this type, the desired lowering of the boiling points is the result of controlled catalytic and thermal reactions, the FCC process being of course carried out so that the cracking unit is in thermal balance. In other words, the feed of hot regenerated catalyst should be such that it is able to meet the various thermal requirements of the reactor section, and more particularly:
Preheating of the liquid feedstock. PA0 Vaporization of the feedstock. PA0 Heat input required by the reactions involved, which overall are endothermic.
The latest developments in the area of catalytic cracking have shown that important factors in the cracking reaction are the rapidity and uniformity with which the feedstock is contacted with the catalyst particles, and hence the quality of the atomization and vaporization of the feedstock upon its injection into the reaction zone. (See U.S. application Ser. No. 827,333 filed Feb. 7, 1986, incorporated herein by reference.)
The feedstocks to be cracked are usually injected into the reaction zone at a temperature which generally ranges from 80.degree. to 400.degree. C. and a relative pressure of from 0.7 to 3.5 bar, while the temperature of the regenerated catalyst fed to that zone may be of the order of 600.degree. to 950.degree. C.
More particularly, when feedstocks composed of hydrocarbons having a high boiling point and containing asphaltenes are being processed, the catalyst is preferably injected into the reaction zone as a fully fluidized suspension at a temperature which may be as high as 950.degree. C., a portion of the quantity of heat so supplied permitting at first the instantaneous thermal cracking of the heaviest and most unstable molecules into lighter molecules. The whole of the feedstock molecules and catalyst particles having by then reached an equilibrium temperature, the catalytic cracking reaction proceeds by contact of these molecules with the active sites of the catalyst.
It has been found that the heavier the hydrocarbon feedstock is, the more critical the configuration of the zone of injection of the feedstock into the catalytic cracking reactor becomes. It should permit optimum contact, that is, uniform and instantaneous contact, of the hydrocarbons with the catalyst. Moreover, practically complete atomization of any unvaporized product should take place as rapidly and as close to the injection zone as possible.
In the most advanced catalytic cracking units now in existence, the atomized hydrocarbon feedstock is injected cocurrently into a stream of catalyst particles circulating in an elongate reaction zone, generally called an elevator or "riser" when the feedstock flows from bottom to top, or a "dropper" when it flows from top to bottom. Regardless of the quality of the atomization of the feedstock, however, the largest droplets are entrained by the catalyst stream and necessarily travel some distance into the reaction zone. The greater that distance, the more the catalyst stream entraining a droplet falls short of carrying the heat necessary for vaporization of that droplet. As a result, the hydrocarbons contained in the largest droplets undergo excessive coking if the droplets collide with catalyst particles, or, if they do not, are converted to a lesser degree into lighter products.